ng curves of those above and those below the mean
corresponding pretty closely with each other. Boot manufacturers, as the
result of experience, construct in effect such a curve, making a large
number of boots of the sizes which in length or breadth are near the
mean, and a symmetrically diminishing number of the sizes above and
below it.

In the next chapter I shall deal with the use in reasoning of such
curves, either actually 'plotted' or roughly imagined. In this chapter I
point out, firstly, that they can be easily remembered (partly because
our visual memory is extremely retentive of the image made by a black
line on a white surface) and that we can in consequence carry in our
minds the quantitative facts as to a number of variations enormously
beyond the possibility of memory if they were treated as isolated
instances; and secondly, that we can by imagining such curves form a
roughly accurate idea of the character of the variations to be expected
as to any inherited quality among groups of individuals not yet born or
not yet measured.

The third and last division under which knowledge of man can be arranged
for the purposes of political study consists of the facts of man's
environment, and of the effect of environment upon his character and
actions. It is the extreme instability and uncertainty of this element
which constitutes the special difficulty of politics. The human type and
the quantitative distribution of its variations are for the politician,
who deals with a few generations only, practically permanent. Man's
environment changes with ever-increasing rapidity. The inherited nature
of every human being varies indeed from that of every other, but the
relative frequency of the most important variations can be forecasted
for each generation. The difference, on the other hand, between one
man's environment and that of other men can be arranged on no curve and
remembered or forecasted by no expedient. Buckle, it is true, attempted
to explain the present and prophesy the future intellectual history of
modern nations by the help of a few generalisations as to the effect of
that small fraction of their environment which consisted of climate. But
Buckle failed, and no one has attacked the problem again with anything
like his confidence.

We can, of course, see that in the environment of any nation or class at
any given time there are some facts which constitute for all its members
a common experience, and there